.\" $OpenBSD: SSL_CTX_set_tlsext_servername_callback.3,v 1.6 2021/09/01 13:56:03 schwarze Exp $
.\" full merge up to: OpenSSL 190b9a03 Jun 28 15:46:13 2017 +0800
.\" selective merge up to: OpenSSL 6328d367 Jul 4 21:58:30 2020 +0200
.\"
.\" This file was written by Jon Spillett <jon.spillett@oracle.com>,
.\" Paul Yang <yang dot yang at baishancloud dot com>, and
.\" Matt Caswell <matt@openssl.org>.
.\" Copyright (c) 2017, 2019 The OpenSSL Project.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\"
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in
.\"    the documentation and/or other materials provided with the
.\"    distribution.
.\"
.\" 3. All advertising materials mentioning features or use of this
.\"    software must display the following acknowledgment:
.\"    "This product includes software developed by the OpenSSL Project
.\"    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
.\"
.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
.\"    endorse or promote products derived from this software without
.\"    prior written permission. For written permission, please contact
.\"    openssl-core@openssl.org.
.\"
.\" 5. Products derived from this software may not be called "OpenSSL"
.\"    nor may "OpenSSL" appear in their names without prior written
.\"    permission of the OpenSSL Project.
.\"
.\" 6. Redistributions of any form whatsoever must retain the following
.\"    acknowledgment:
.\"    "This product includes software developed by the OpenSSL Project
.\"    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
.\" OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: September 1 2021 $
.Dt SSL_CTX_SET_TLSEXT_SERVERNAME_CALLBACK 3
.Os
.Sh NAME
.Nm SSL_CTX_set_tlsext_servername_callback ,
.Nm SSL_CTX_set_tlsext_servername_arg ,
.Nm SSL_get_servername_type ,
.Nm SSL_get_servername ,
.Nm SSL_set_tlsext_host_name
.Nd handle server name indication (SNI)
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft long
.Fo SSL_CTX_set_tlsext_servername_callback
.Fa "SSL_CTX *ctx"
.Fa "int (*cb)(SSL *ssl, int *alert, void *arg)"
.Fc
.Ft long
.Fo SSL_CTX_set_tlsext_servername_arg
.Fa "SSL_CTX *ctx"
.Fa "void *arg"
.Fc
.Ft const char *
.Fo SSL_get_servername
.Fa "const SSL *ssl"
.Fa "const int type"
.Fc
.Ft int
.Fo SSL_get_servername_type
.Fa "const SSL *ssl"
.Fc
.Ft int
.Fo SSL_set_tlsext_host_name
.Fa "const SSL *ssl"
.Fa "const char *name"
.Fc
.Sh DESCRIPTION
.Fn SSL_CTX_set_tlsext_servername_callback
sets the application callback
.Fa cb
used by a server to perform any actions or configuration required based
on the servername extension received in the incoming connection.
Like the ALPN callback, it is executed during Client Hello processing.
When
.Fa cb
is
.Dv NULL ,
SNI is not used.
.Pp
The servername callback should return one of the following values:
.Bl -tag -width Ds
.It Dv SSL_TLSEXT_ERR_OK
This is used to indicate that the servername requested by the client
has been accepted.
Typically a server will call
.Xr SSL_set_SSL_CTX 3
in the callback to set up a different configuration
for the selected servername in this case.
.It Dv SSL_TLSEXT_ERR_ALERT_FATAL
In this case the servername requested by the client is not accepted
and the handshake will be aborted.
The value of the alert to be used should be stored in the location
pointed to by the
.Fa alert
parameter to the callback.
By default this value is initialised to
.Dv SSL_AD_UNRECOGNIZED_NAME .
.It Dv SSL_TLSEXT_ERR_ALERT_WARNING
If this value is returned, then the servername is not accepted by the server.
However, the handshake will continue and send a warning alert instead.
The value of the alert should be stored in the location pointed to by the
.Fa alert
parameter as for
.Dv SSL_TLSEXT_ERR_ALERT_FATAL
above.
Note that TLSv1.3 does not support warning alerts, so if TLSv1.3 has
been negotiated then this return value is treated the same way as
.Dv SSL_TLSEXT_ERR_NOACK .
.It Dv SSL_TLSEXT_ERR_NOACK
This return value indicates
that the servername is not accepted by the server.
No alerts are sent
and the server will not acknowledge the requested servername.
.El
.Pp
.Fn SSL_CTX_set_tlsext_servername_arg
sets a context-specific argument to be passed into the callback via the
.Fa arg
parameter for
.Fa ctx .
.ig end_of_get_servername_details
.\" I would suggest to comment out that second wall text of dubious
.\" usefulness and see if we can meet all these documented API
.\" requirements in the future or decide that it's not worth the
.\" effort.  -- tb@ Aug 30, 2021
.Pp
The behaviour of
.Fn SSL_get_servername
depends on a number of different factors.
In particular note that in TLSv1.3,
the servername is negotiated in every handshake.
In TLSv1.2 the servername is only negotiated on initial handshakes
and not on resumption handshakes.
.Bl -tag -width Ds
.It On the client, before the handshake:
If a servername has been set via a call to
.Fn SSL_set_tlsext_host_name ,
then it will return that servername.
If one has not been set, but a TLSv1.2 resumption is being attempted
and the session from the original handshake had a servername
accepted by the server, then it will return that servername.
Otherwise it returns
.Dv NULL .
.It On the client, during or after the handshake,\
 if a TLSv1.2 (or below) resumption occurred:
If the session from the original handshake had a servername accepted by the
server, then it will return that servername.
Otherwise it returns the servername set via
.Fn SSL_set_tlsext_host_name
or
.Dv NULL
if it was not called.
.It On the client, during or after the handshake,\
 if a TLSv1.2 (or below) resumption did not occur:
It will return the servername set via
.Fn SSL_set_tlsext_host_name
or
.Dv NULL
if it was not called.
.It On the server, before the handshake:
The function will always return
.Dv NULL
before the handshake.
.It On the server, after the servername extension has been processed,\
 if a TLSv1.2 (or below) resumption occurred:
If a servername was accepted by the server in the original handshake,
then it will return that servername, or
.Dv NULL
otherwise.
.It On the server, after the servername extension has been processed,\
 if a TLSv1.2 (or below) resumption did not occur:
The function will return the servername
requested by the client in this handshake or
.Dv NULL
if none was requested.
.El
.Pp
Note that the early callback occurs before a servername extension
from the client is processed.
The servername, certificate and ALPN callbacks occur
after a servername extension from the client is processed.
.end_of_get_servername_details
.Pp
.Fn SSL_set_tlsext_host_name
sets the server name indication ClientHello extension
to contain the value
.Fa name ,
or clears it if
.Fa name
is
.Dv NULL .
The type of server name indication
extension is set to
.Dv TLSEXT_NAMETYPE_host_name
as defined in RFC 3546.
.Pp
All three functions are implemented as macros.
.Sh RETURN VALUES
.Fn SSL_CTX_set_tlsext_servername_callback
and
.Fn SSL_CTX_set_tlsext_servername_arg
always return 1 indicating success.
.Pp
.Fn SSL_get_servername
returns a servername extension value of the specified type if provided
in the Client Hello, or
.Dv NULL
otherwise.
.Pp
.Fn SSL_get_servername_type
returns the servername type or -1 if no servername is present.
Currently the only supported type (defined in RFC 3546) is
.Dv TLSEXT_NAMETYPE_host_name .
.Pp
.Fn SSL_set_tlsext_host_name
returns 1 on success or 0 in case of an error.
.Sh SEE ALSO
.Xr ssl 3 ,
.Xr SSL_CTX_callback_ctrl 3 ,
.Xr SSL_CTX_set_alpn_select_cb 3
.Sh HISTORY
These functions first appeared in OpenSSL 0.9.8f
and have been available since
.Ox 4.5 .
